It is conventional in the offshore pipeline industry to use weight coated pipe for pipelines which are used on ocean floors or other underwater surfaces. The weight coats traditionally have been made of dense materials such as concrete, and are typically several inches thick around the circumference of the pipe. The weight coats protect the pipeline and provide sufficient weight to maintain the pipeline submerged in a non-buoyant condition.
In most cases, the weight coats are applied to the full length of the pipe except for a short distance where there is a bare pipe end portion, approximately one foot from the end of each pipe section. The end portion of the pipe remains without the weight coat to facilitate welding together individual sections of the weight coated pipe in order to make up the pipeline. In this manner, sections of pipe are placed on a barge and welded sequentially onto preceding sections forming a pipeline extending from the barge. The newly formed pipeline is placed on rollers, and as the barge moves forward, the pipeline is carried over the rollers, then lowered, and then laid on the bed of the body of water.
The portions of pipe not having a weight coat had a corrosion coating applied to the surface of the pipe to prevent the pipe from corroding due to exposure to the elements. Generally, the corrosion coatings used were a heat shrinking tape or a fusion bonded epoxy. After the sections of pipe were welded together, various techniques were used to protect the corrosion coating on the exposed portions of pipe around each joint.
One prior known procedure was to wrap sheet metal over the weight coating adjacent the exposed portion of the pipe and band the sheet metal in place with metal bands. Generally, a zinc coated sheet metal was used. The space between the pipe and sheet metal was then filled with a molten material which would solidify as it cooled. However, in most cases, the pipeline had to be in a condition for handling immediately after the sleeves were filled so that the laying of the pipeline could proceed without delay. The molten filling did not set or harden to a sufficiently strong material within the required time to allow further processing of the pipe and the molten material would leach out into the water if the pipeline was lowered before the molten material was adequately cured.
Other known procedures have typically replaced the molten material with other types of materials. For example, one alternative material utilized to cover the exposed portion of pipe was granular or particulate matter such as gravel or iron ore which did not pack solidly or uniformly. Then elastomeric polyurethanes were injected into the mold to fill the interstices between the granular filler materials. After the polymer material had reacted, the mold would be removed from the surface of the infill.
Another known procedure involves wrapping the exposed portions of pipe with a thermoplastic sheet. The sheet overlapped the ends of the weight coat adjacent the exposed joint and then was secured in place by screws, rivets, or straps. To increase the rigidity and impact resistance, this joint protection system required the installation of reinforcing members such as plastic bars or tubes to the interior of the sheet. The reinforcement bars or tubes either had to be precut and stored on the barge or else cut to the required fitting form as part of the installation process on the barge. Yet another known procedure entailed filling the lower portion of the annular space between the pipe and the plastic sheet with a material such as pre-formed foam half shells.
A more recently used technique involved encasing the pipe joint by circumferentially wrapping a pliable sheet of cover material around the exposed portion of the joint connection. The longitudinal end portions of the pliable cover overlapped the adjacent edges of the weight coating, such that an annular pocket was formed about the exposed joint section. Polyurethane forming chemicals were then injected into the empty annular space where they reacted to form high-density, open cell foam which filled the annular space. The open cell polyurethane foam was intended to absorb moisture and ultimately increase the ballast of the pipeline.
In many cases, vibrations during offshore operations at times could cause the foam to vibrate, and move around, tending to separate the foam from the pipe, because there was no locking mechanism to hold the polyurethane foam securely in its place. Of further concern, the outer diameter portions of the foam were more susceptible to movement, agitation, or damage than the inner diameter portions of the foam, because the outer diameter portions might have a lower density than the inner diameter portions of the foam.